Underground coal gasification is a process by which product gas is produced from a coal seam by heating the coal in situ in the presence of an oxidant (e.g., air or oxygen). The product gas is typically referred to as synthesis gas or syngas and can be used, for example, as a feedstock for various applications, including clean fuels production, chemical production, and electricity generation.
Conversion of coal into product gas takes place in a well that includes one or more bore holes drilled into the coal seam that are in fluidic communication with one another. Typically, that part of the well into which the oxidant is introduced to promote combustion is called an injection well and that part of the well from which product gas is withdrawn is called a production well. Both horizontal and vertical wells can be used for oxidant injection and product gas production.
A coal seam panel is typically referred to as a coal gasifier. Gasification occurs adjacent a combustion zone of the well/gasifier and the coal is partially oxidized to produce product gas of low or medium heating value. Hot product gas flows from the gasification zone and exits the ground via a well head of the production well.
Some of the major challenges in UCG include preventing blockage to the production well due to coal spalling, slugging or the ingress of water, controlling interaction of the product gas with the coal seam in well zones in which chemical reactions have not yet taken place (i.e., cold zones), reigniting the coal seam, and preventing the escape of product gas into the surrounding environment.